Membrane Physiology

Dr Sadia Amer

Learning Objectives To know about the method of

recording of current flow through single channel.• To know about facilitated diffusion.• To know about the factors affecting

net rate of diffusion.

Open state versus closed state of Gating Channels

• It shows two recording of electrical current flowing through a single Na channel when there was an approximate 25 mv potential gradient across the membrane.

• This shows that channel conducts current either “ all or none”.• The gate of the channel snaps open

and then snaps closed, each open state last for only a fraction of a millisecond upto several milliseconds.

• At one voltage potential level, channel may remain closed all the time or almost all the time , whereas at another voltage level, it may remain open either all or most of the time.

Patch clamp method for recording ion current flow

through single channel

• Micropipette- tip diameter- 1 to2mm

Facilitated Diffusion• In it transported substance diffuses

through the membrane using a specific protein and this carrier facilitates diffusion.• It is also called Carrier Mediated

Diffusion.

• Difference between simple and facilitated diffusion is of Vmax level.• It means the rate of diffusion cannot

rise beyond a certain level.

• Because the binding force of receptor is weak , the thermal motion of attached molecule causes it to break away an to be released on the opposite side of membrane.

• The rate at which molecule can be transported can never be greater than the rate at which the carrier protein molecule can undergo change back and forth b/w its two states.

• Examples of facilitated diffusion are GLUCOSE and most of AMINOACIDS.• Carrier molecule with molecular wt

45,000 has been discovered for glucose.

• Several monosaccharides having glucose like structures are transported like galactose.• Insulin increases rate of diffusion by

10 to 20 folds.

Factors affecting net rate of diffusion

Effect of concentration differenceEffect of membrane electrical

potentialEffect of pressure difference

Effect of conc. differenceThe rate of net diffusion into cell is

proportional to the conc. On the outside minus conc. On the inside.

Net diffusion~ (Co-Ci)

Co is conc. Outside

Ci is conc. inside

Effect of electrical potential• At normal body temp. the electrical

difference that will balance a given conc. difference of univalent ions such as Na ions can be determined from formula called Nerst equation:-• EMF(in mv)=61logC1/C2

Effect of pressure difference• Pressure means sum of all the forces

of different molecules striking a unit surface area at a given instant.

Osmosis• Diffusion of substances across a

semipermeable membrane against the concentration gradient is called osmosis.

Osmotic Pressure• The exact amount of pressure

required to stop osmosis is called the osmotic pressure.

Importance of number of osmotic particles in

determining osmotic pressure

• Osmotic pressure is determined by number of particles per unit volume of fluid , not by mass of particles.• The large particles have greater mass

but they move at slow velocities.• The small particles move at high

velocity.

• Their kinetic energy is determined by:-• k=mv2/2• It is same for small as well as for

large particles.• So number of particles is important.